1. Technical Field
The disclosed embodiments generally relate to the field of conveyor belt weighing systems. More particularly, the description generally relates to generators as a means for providing excitation voltage for conveyor belt scales, and methods for using generators with conveyor belt scales.
2. Description of the Related Art
It is often desirable to measure and/or control the mass flow rate of material such as aggregate moving on a conveyor belt. By combining the weight of the material and the speed of the conveyor belt, a continuous indication of the mass flow rate of the material moving on the conveyor belt can be generated. Conveyor belt scales are integrated weighting devices that use one or more algorithms, such as a simple integral calculus summation process, to measure a conveyed quantity of material according to at least two variables: weight and speed. Both the need for totalized weight and the need for flow rate require similar components for the belt scale: a mechanical structure equipped with one or more weight measuring devices that supports a short section of the loaded belt and produces a signal indicative of the magnitude of the load, a belt motion measuring device, and an electronic signal processor that combines the weight and motion signals and computes totalized weight, belt speed, and material weight flow rate. The belt scale generally plays the role of a flow rate measurement. A separate plant control system may use the measurement information to change the flow rate as needed to meet momentary and changing demands.
A conveyor belt system typically includes one or more idlers connected mechanically to one or more load cells. An endless belt is looped around the idlers and the head and tail pulleys. One of the pulleys is driven so that the belt is continuously moving over the idlers. An electric motor is commonly used as the prime mover of a conveyor belt system, or any portions therein.
In the prior art, there are several devices and methods for detecting the weight of a load on a conveyor belt. Conventional belt scales include load cells situated below a conveyor belt that generate an output signal proportional to the weight of a load moving across a conveyor belt. See, for example, U.S. Pat. Nos. 3,478,830; 3,439,761; 3,785,447; 3,924,729; 4,682,664; 4,788,930; 4,463,816; and 4,557,341, the disclosures of each of which are incorporated herein by reference in their entirety. Many of the prior art devices utilize a mechanical weighframe or idler support structure, which Supports a section of the conveyor belt. A portion of the weighframe is often coupled to a strain gauge load sensors or load cell, where the strain gauge deforms proportionately to the load on the belt. The gauge's resistance varies as it deforms and by applying a voltage across the gauge, an electrical signal is generated which is proportionate to the weight of the load on the belt at a given instant in time. The load cell is a mechanical force to electrical signal transducer where any weight applied to the weighframe is transferred to the load cell for measurement. The weight signal that comes from the scale is a voltage having an amplitude proportional to the weight per length on the conveyor belt. If the weight on the belt increases, the load cell delivers a higher electrical signal, and as the weight decreases, it delivers a lower electrical signal. Although the load cell deforms proportionately to the load on the belt, the load on a moving belt generates forces in both a forward or horizontal direction (i.e. tangential forces caused from the moving belt) as well as a downward or vertical direction (i.e. the weight of the load). In conventional belt scales, a single load cell is typically located below the belt such that when the belt stops moving and the forward forces cease to exist, the downward forces measured by the load cell increase, thereby causing inaccuracies in the actual weight measurements.
U.S. Pat. No. 5,294,756, the disclosure of which is incorporated herein by reference in its entirety, describes a scale apparatus for weighing material moving on a conveyor belt supported by an idler assembly. The apparatus includes a load cell support and a load cell. The load cell includes a base and a contact portion extending upwardly away from the base. The base of the load cell is configured to bend in response to a vertical force being applied to the contact portion to generate an output signal proportional to the fore. The load cell is non-linear responsive to horizontal forces, which is best described as off-axis. The idler support applies a downwardly directed force to the contact portion of the load cell to bend the base of the load cell in response to material moving on the conveyor belt over the idler support to change the output signal generated by the load cell in proportion to the weight of the material. The magnitude of the force applied by the idler support changes as the weight of the material moving over the idler support changes.
The speed of the belt is another variable measured by conventional conveyor belt scales. Most conventional speed sensors, commonly referred to as “encoders,” are rotary digital pulse generators, which can be optical, magnetic or other or off sensing units. They are typically mounted on a pulley or wheel that rotates as the belt moves, generating an on/off signal as they move that is directly proportional to the distance the belt moves and the speed of the belt. Generally, the encoder transmits more pulses as the belt speed increases, and less pulses as the belt speed decreases. These units typically mount to the non-drive end of the conveyor. U.S. Pat. No. RE 29,944, the disclosure of which is incorporated herein by reference in its entirety for example, describes a belt travel pulse generator that is coupled to a pulley that produces an output belt travel signal comprising a sequence of pulses on an electrical line. The belt travel signal pulses are generated by movement of the belt, or they may be generated by the power frequency if the drive means comprises a synchronous motor. The repetition rate of the pulses on the line is directly proportional to the belt velocity past the pulley. For example, the generator may generate one hundred pulses per revolution of the pulley which can be converted mathematically to be directly proportional to the belt speed in feet per minute.
Many applications of belt scale conveyor systems incorporate an electronic integrator which receives weight-related signals through an electrical wire from the belt scale and a belt speed signal through an electrical wire from the encoder. The integrator integrates the product of these two signals and provides an output signal which is indicative of the weight of material that passes on the portion of the belt associated with the scale input sensor. Electronic integrators of this type are well known in the art. U.S. Pat. No. 3,610,908, the disclosure of which is incorporated herein by reference in its entirety, for example, describes a solid-state electronic integrator system that includes dipout pulses to afford continuous running and automatic self-compensation with respect to control signal variation. This system receives variable amplitude flowing weight signals and variable frequency speed signals and integrates them, providing a digital signal that is a function of the gravimetric flow rate. U.S. Pat. No. 3,559,451, the disclosure of which is incorporated herein by reference in its entirety, describes a totalizing and flow rate measuring system which includes an integrator of the type noted above to generate a digital weight signal which is subsequently processed to produce output signals or indications representative of the cumulative weight and the instantaneous flow rate of material on the belt which passes the input sensor of the scale.
Newer systems are making a transition to a fully digital approach, where a microprocessor often takes over both speed control and electronic commutation. The controller or microprocessor draws its current or power from the electric lines, provides an excitation voltage for the load cell, receives the variable voltage from the output of the load cell, provides excitation voltage for the encoder and receives the signal from the encoder. It is desirable to minimize the amount of control electronics needed to run the belt scale system, the number of lines running between the controller and the load cell and encoder. Relying on voltage and signal wires from the load cell and encoder makes the systems more susceptible to lightning strikes.
The disclosure contained herein describes attempts to address one or more of the problems described above.